1. Field of the Invention
This invention relates to charge coupled device (CCD) image sensors.
More particularly the invention relates to CCD image sensors of frame transfer format.
2. Description of Related Art
A frame transfer CCD image sensor of conventional form will now be described with reference to FIGS. 1, 2 and 3 of the accompanying drawings which are diagrams illustrating the sensor and its operation.
Referring to FIG. 1, the sensor includes an array 1 of similar semiconductor electric charge storage and transfer channels formed on a substrate and arranged side by side, the channels extending in the vertical direction in FIG. 1.
Referring now to FIG. 2, each channel comprises a number of charge storage locations defined by an associated electrode structure 3 (not shown in FIG. 1), overlying an insulating layer 2 formed on the substrate 4. With appropriately differing potentials applied to the electrodes of the electrode structure 3 the maximum potential in the substrate, as indicated by line 5 in FIG. 2, varies along the length of the channel, thus defining potential wells 7 under some of the electrodes 3, e.g. with a three phase electrode structure, under every third electrode 3, as shown in FIG. 1. In these potential wells 7 any electric charge of appropriate polarity which is present will accumulate. By appropriately cycling the potentials applied to the electrodes 3, the potential wells 7, and hence any charge therein, may be transferred along the channel.
One half of the array (the upper half in FIG. 1) is open to light and forms an image section 9 of the array 1. The other half of the array 1 is shielded from light and forms a store section 11 of the array 1.
At the end of the store section 11 remote from the image section 9 there is a read-out section 13 of the sensor constituted by a further charge storage and transfer channel extending transverse to the channels of the store section 11. An amplifier 15 amplifies the electrical output signal of the read-out section 13.
In operation of the sensor an optical image to be converted into an electrical signal is focussed onto the image section 9. The incident light causes electric charges to be generated and stored in the image section 9 in a pattern corresponding to the image. This charge pattern is then quickly transferred to the store section 11, as indicated by the arrow 17 in FIG. 1. Whilst a further charge pattern collects in the image section 9, the charge pattern in the store section 11 is transferred to, and read out by the read-out section 13, line by line, to form an output electric signal representing the image. It will be appreciated that each line of the charge pattern is constituted by the charges in a different set of corresponding charge storage locations in the side by side channels.
A fundamental limitation of conventional frame transfer image sensors is that spurious charges are photogenerated in the image section of the array as the charge pattern corresponding to an image is being transferred from the image section to the store section.
For example, referring again to FIG. 1, if the image focussed on the image section 9 is a bright spot, as indicated at 19 in FIG. 1, when the charge pattern generated and stored in the image section 9 is transferred to the store section 11, the potential wells transferred through the region of bright spot 19 will pick up spurious charge (so-called frame shift smear) in direct proportion to the intensity of illumination of spot 19 and in inverse proportion to the rate of charge transfer. At the end of a first charge transfer, spurious charges will be present in the store section 11 in all the nominally empty potential wells above and in line with the position of the charges truly representing the bright spot, and will be present in the image section 9 in all the nominally empty potential walls below and in line with the position of the bright spot 19, as indicated by the dotted lines 21 in FIG. 1. At the end of the next charge transfer spurious charges will again be present in the image section 9, as at the end of the first charge transfer. However, in the store section 11 spurious charges will now be present both above and below the position 19' of the charges truly representing the bright spot, the charges above having been created during the second transfer, and those below being those created in the image section 9 during the first transfer and transferred to the store section 11 during the second charge transfer.
The net result is therefore that a spurious output signal appears for all points above and below the bright spot 19 giving in effect a vertical ghost line 23 in an image recreated from the output signals, as indicated in FIG. 3.